This invention relates to composite springs and, more particularly, to composite springs made up of natural rubber or other elastomer in combination with a coil spring or other reinforcement. As used herein, the term "stiff" refers to those portions of the force versus deflection curve of the composite spring that are of relatively high spring rate, whether constant slope or generally rising rate and is characteristic of progressively increasing resistance to compressive deformation. The term "soft" refers to and is characteristic of relatively lower spring rate or less resistance to compressive deformation, whether increasing, decreasing or none at all.
In the past, composite springs of this type have provided force/deflection curves in which the spring is soft near the middle of the curve and is stiff at each end. This result is obtained by controlling deformation of the elastomer spring element under certain conditions in which it is deformable according to different but essentially stable characteristics. One example of such a composite spring is found in the U.S. Pat. No. 2,605,099 by Brown. This composite spring is made up of a rubber envelope that has an undulatory wall section reinforced by and bonded to a steel spring. Another generally similar composite spring is found in U.S. Pat. No. 2,822,165 by Boschi.
The principle drawback of these composite springs is that the soft region of the force/deflection curve, if any, is of very limited duration. This is unsatisfactory in some applications in which it is desireable to have a soft region of extended duration. One, but not the only such application, is for vehicular suspension systems and, in particular, suspension systems for automotive vehicles. This is so because ride comfort often is associated with the ride characteristics that are derived from the soft region of the force/deflection curve of the suspension springs.